The present invention generally relates to semiconductor devices and more particularly to a ferroelectric capacitor for use in a semiconductor device and a semiconductor device having such a ferroelectric capacitor. Further, the present invention relates to the fabrication process of such a semiconductor device.
Semiconductor memory devices such as DRAMs or SRAMs are used extensively in various information processing apparatuses including computer as a high-speed main memory device. On the other and, DRAMs or SRAMs are volatile in nature and the information stored therein is lost when the electric power is turned off. Thus, it has been practiced to use non-volatile magnetic disk devices as a large-capacity storage device for storing programs and data.
On the other hand, magnetic disk devices have various drawbacks in that they are bulky and mechanically fragile. Further, they consume large electric power and provide a poor access speed when reading or writing information. Thus, the use of magnetic devices have been limited to the large-capacity auxiliary storage device.
In view of the drawbacks of the magnetic disk devices, there is an increasing tendency of using an EEPROM or flash memory device as a non-volatile memory device, wherein an EEPROM or a flash memory device stores information in a floating gate electrode in the form of electric charges. Particularly, flash memory devices, having a memory cell construction similar to that of a DRAM, is suitable for constructing an integrated circuit having a very large integration density and is expected to play a major role in a large-storage device having a storage capacity comparable to that of a magnetic disk device.
On the other hand, such EEPROM or flash memory device still suffers from the problem of low access speed due to the operational principle of the device in that writing of information is achieved by injection of hot electrons into the floating gate electrode through a tunneling oxide film. Further, there arises a problem, in such an EEPROM or flash memory device, of deterioration of the tunneling insulation film when writing or erasing of information is conducted repeatedly over a prolonged time period. When the quality of the tunneling insulation film is deteriorated, writing or erasing operation of the device becomes unstable.
Meanwhile, there is a proposal of a ferroelectric semiconductor memory device referred to hereinafter as FeRAM that stores information in the form of spontaneous polarization of a ferroelectric film. An FeRAM has a construction similar to that of a DRAM in that each memory cell transistor is formed of a single MOSFET, except that the capacitor dielectric film of the memory cell capacitor is replaced with a ferroelectric film such as PZT (Pb(Zr,Ti)O3), PLZT (Pb(Zr,Ti,La)O3, SBT (SrBi2Ta2O3), and the like. Thus, an FeRAM is suitable for constructing a large-scale integrated circuit similarly to a DRAM. Further, an FeRAM has an advantageous feature, due to the operational principle thereof in that the spontaneous polarization of the ferroelectric capacitor is caused by application of an electric field, in that the writing speed is improved by the factor of 1,000 or more as compared with the writing speed of an EEPROM or a flash memory device and that the electric power consumption is reduced by a factor of 10. Further, in view of the fact that the device uses no tunneling insulation film, an FeRAM is expected to allow repetition of writing information by a factor of 100,000 as compared with a flash memory device.
In a typical ferroelectric capacitor used in such an FeRAM, a ferroelectric film of PZT or PLZT is deposited on a lower electrode of Pt by a sputtering process. The ferroelectric film thus deposited is subjected to a crystallization process in an oxidizing atmosphere, and an upper electrode of Pt is deposited on the ferroelectric film thus processed. It should be noted that the ferroelectric film is amorphous right after the deposition process, and it is inevitable to conduct a crystallization process by a thermal annealing process in order to obtain a desired ferroelectric property. By conducting the thermal annealing process in the oxidizing atmosphere, the oxygen defects in the ferroelectric film are effectively compensated for at the time of the crystallization process.
In the fabrication process of a semiconductor device having such a ferroelectric capacitor, it is generally inevitable to cover the ferroelectric capacitor by depositing an interlayer insulation film or a passivation film, similarly to other semiconductor devices.
Generally, the deposition of such an interlayer insulation film is conducted by a CVD process while using a silane compound such as SiH4 or Si2H6 as a gaseous source material. On the other hand, the CVD process using such a silane compound is conducted generally in a reducing atmosphere containing excessive H2, and thus, there arises a problem in that the ferroelectric film constituting the ferroelectric capacitor is exposed to the reducing atmosphere. Thereby, the ferroelectric film easily undergoes a reduction and there again appear oxygen defects. As a result of oxygen defect formation, the spontaneous polarization or switching electric charge of the ferroelectric film is reduced substantially and the electrical performance of the ferroelectric capacitor is seriously deteriorated.
In order to attend to this problem of reduction of the ferroelectric film at the time of formation of the interlayer insulation film, Japanese Laid-Open Patent Publication 6-290984 proposes a construction in which the upper electrode is covered by a protective film of various oxides including Al2O3. However, the foregoing conventional proposal has not been sufficient for eliminating the reduction of the ferroelectric film and degradation of performance of the ferroelectric capacitor.
Accordingly, it is a general object of the present invention to provide a novel and useful ferroelectric capacitor and a semiconductor device having such a ferroelectric capacitor wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a ferroelectric capacitor and a semiconductor device having such a ferroelectric capacitor wherein the problem of degradation of the ferroelectric capacitor at the time of thermal annealing process conducted in a reducing atmosphere is successfully eliminated.
Another object of the present invention is to provide a semiconductor device, comprising:
a ferroelectric capacitor; and
a protective film of Al2O3 covering said ferroelectric capacitor,
said ferroelectric capacitor comprising a lower electrode, a ferroelectric insulation film formed on said lower electrode, and an upper electrode formed on said ferroelectric insulation film,
wherein said protective film has a density exceeding 2.7 g/cm3 when said protective film has a thickness exceeding about 20 nm,
said protective film has a density exceeding 3.0 g/cm3 when said protective film has a thickness less than about 20 nm.
Another object of the present invention is to provide a semiconductor device, comprising:
a ferroelectric capacitor; and
a protective film of Al2O3 covering said ferroelectric capacitor,
said ferroelectric capacitor comprising a lower electrode, a ferroelectric insulation film formed on said lower electrode, and an upper electrode formed on said ferroelectric insulation film,
wherein said protective film shows an etching rate, when having a thickness exceeding about 20 nm, of less than 100 nm/min against a 10% diluted buffered HF etching solution containing 4.9% of NH4F and 0.7% of HF,
said protective film shows an etching rate, when having a thickness less than about 20 nm, of less than 50 nm/min against said buffered HF etching solution.
Another object of the present invention is to provide a method of fabricating a semiconductor device having a ferroelectric capacitor, comprising the step of:
covering said ferroelectric capacitor by a protective film of Al2O3,
wherein said protective film is deposited by any of an electron cyclotron resonant plasma sputtering process or an induction-coupled high-frequency-plasma-assisted magnetron sputtering process.
According to the present invention, a dense protective film of Al2O3 is formed on the ferroelectric capacitor and the penetration of H2 into the ferroelectric capacitor insulation film is effectively blocked. Thereby, a large switching electric charge is guaranteed for the ferroelectric capacitor insulation film.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.